The Group of Seven biggest industrial nations and the European Union have agreed to double their promised aid to $600 million to help rebuild the leaky cement shell that covers the ruined reactor at Chernobyl, a German official said Tuesday.The additional aid, to be announced Wednesday at a donor conference convened by German Chancellor Gerhard Schroeder in Berlin, means Ukraine will have almost the entire $768 million it says is needed for the project, the official said on condition of anonymity.